Processes for measuring the concentration of gases with thermal measuring elements are known per se. As a rule, the thermal behavior of the measuring element is observed by evaluating electric measured variables. The thermal behavior of the measuring element depends in different ways on the concentrations of different gases in the environment of the measuring element.
It is known that such processes can be carried out with so-called pellistors, which are partially catalytically active (U.S. Pat. No. 4,457,954, GB 2083630, U.S. Pat. No. 4,583,070). Two pellistors are usually used in case of the catalytic principle of measurement, one pellistor being prepared catalytically, while the second pellistor does not have this catalytic preparation.
The behavior of the catalytically prepared pellistor to change its resistance, which is characteristic of the gas to be detected, compared to the second, unprepared pellistor can be evaluated by means of a prior-art resistance bridge.
Various operating processes, such as constant-current, constant-voltage or constant-resistance processes, are known per se. The drawback of these processes in continuous operation is the high power consumption, which may be between 250 mW and 700 mW.
The following patent specifications U.S. Pat. No. 4,861,557, DE 4330603 and DE 3131710 describe processes with a pellistor bridge. The drawbacks of such processes are the complicated apparatus required for two measuring elements, the actuation thereof in a continuous mode of operation, and the high power requirement associated therewith.
A process with low power consumption is known, in which combustible gases are detected with only one measuring element (EP 0234251 A1). The gas concentration is determined in two stationary measurement phases according to this process.
The drawbacks of this process are that interfering environmental effects, e.g., temperature, pressure or moisture, are not compensated, and the power consumption is still very high for the operation in two measurement phases.
It is known that thermal measuring elements can be operated cyclically, in which case three different phases of operation alternate regularly (DE 69020343 T2). A heating phase is first carried out, during which the measuring element is heated to a preset resistance. This is followed by the measurement phase, during which the measuring element is maintained at a constant resistance value. This is followed by a rest phase, during which the measuring element is adjusted to a static resistance. A Pt air-core coil with a wire diameter of 80 μm is mentioned as the measuring element. The working temperature of the resistor element is therefore selected to be such that the temperature set point of the Pt coil is in the range of 570° C. to 1,100° C. By contrast, the use of pellistors was rejected, mainly because of the thermal inertia of the pellistor beads and the limited long-term stability. The drawback of this process is that the Pt air-core coils also have an excessively low long-term stability at such high operating temperatures and a relatively high power consumption is also associated with the high operating temperature.
Furthermore, it is known that the impression of an excitation function and evaluation of the response function can be used to determine the percentage of combustible gases in a gas mixture as well as to classify them according to gas classes (DE 4311605 C1). Depending on the selected function, the effort needed for control and evaluation may be high.